/*
 * Copyright (C) 2010 ZXing authors
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package com.google.zxing.common;

import java.util.Map;

import com.google.zxing.DecodeHintType;

/**
 * Common string-related functions.
 *
 * @author Sean Owen
 */
public final class StringUtils {

    private static final String PLATFORM_DEFAULT_ENCODING =
            System.getProperty("file.encoding");
    public static final String SHIFT_JIS = "SJIS";
    public static final String GB2312 = "GB2312";
    private static final String EUC_JP = "EUC_JP";
    private static final String UTF8 = "UTF8";
    private static final String ISO88591 = "ISO8859_1";
    private static final boolean ASSUME_SHIFT_JIS =
            SHIFT_JIS.equalsIgnoreCase(PLATFORM_DEFAULT_ENCODING) ||
                    EUC_JP.equalsIgnoreCase(PLATFORM_DEFAULT_ENCODING);

    private StringUtils() {
    }

    /**
     * @param bytes bytes encoding a string, whose encoding should be guessed
     * @param hints decode hints if applicable
     * @return name of guessed encoding; at the moment will only guess one of:
     * {@link #SHIFT_JIS}, {@link #UTF8}, {@link #ISO88591}, or the platform
     * default encoding if none of these can possibly be correct
     */
    public static String guessEncoding(byte[] bytes, Map<DecodeHintType, ?> hints) {
        if (hints != null) {
            String characterSet = (String) hints.get(DecodeHintType.CHARACTER_SET);
            if (characterSet != null) {
                return characterSet;
            }
        }
        // Does it start with the UTF-8 byte order mark? then guess it's UTF-8
        if (bytes.length > 3 &&
                bytes[0] == (byte) 0xEF &&
                bytes[1] == (byte) 0xBB &&
                bytes[2] == (byte) 0xBF) {
            return UTF8;
        }
        // For now, merely tries to distinguish ISO-8859-1, UTF-8 and Shift_JIS,
        // which should be by far the most common encodings. ISO-8859-1
        // should not have bytes in the 0x80 - 0x9F range, while Shift_JIS
        // uses this as a first byte of a two-byte character. If we see this
        // followed by a valid second byte in Shift_JIS, assume it is Shift_JIS.
        // If we see something else in that second byte, we'll make the risky guess
        // that it's UTF-8.
        int length = bytes.length;
        boolean canBeISO88591 = true;
        boolean canBeShiftJIS = true;
        boolean canBeUTF8 = true;
        int utf8BytesLeft = 0;
        int maybeDoubleByteCount = 0;
        int maybeSingleByteKatakanaCount = 0;
        boolean sawLatin1Supplement = false;
        boolean sawUTF8Start = false;
        boolean lastWasPossibleDoubleByteStart = false;

        for (int i = 0;
             i < length && (canBeISO88591 || canBeShiftJIS || canBeUTF8);
             i++) {

            int value = bytes[i] & 0xFF;

            // UTF-8 stuff
            if (value >= 0x80 && value <= 0xBF) {
                if (utf8BytesLeft > 0) {
                    utf8BytesLeft--;
                }
            } else {
                if (utf8BytesLeft > 0) {
                    canBeUTF8 = false;
                }
                if (value >= 0xC0 && value <= 0xFD) {
                    sawUTF8Start = true;
                    int valueCopy = value;
                    while ((valueCopy & 0x40) != 0) {
                        utf8BytesLeft++;
                        valueCopy <<= 1;
                    }
                }
            }

            // ISO-8859-1 stuff

            if ((value == 0xC2 || value == 0xC3) && i < length - 1) {
                // This is really a poor hack. The slightly more exotic characters people might want to put in
                // a QR Code, by which I mean the Latin-1 supplement characters (e.g. u-umlaut) have encodings
                // that start with 0xC2 followed by [0xA0,0xBF], or start with 0xC3 followed by [0x80,0xBF].
                int nextValue = bytes[i + 1] & 0xFF;
                if (nextValue <= 0xBF &&
                        ((value == 0xC2 && nextValue >= 0xA0) || (value == 0xC3 && nextValue >= 0x80))) {
                    sawLatin1Supplement = true;
                }
            }
            if (value >= 0x7F && value <= 0x9F) {
                canBeISO88591 = false;
            }

            // Shift_JIS stuff

            if (value >= 0xA1 && value <= 0xDF) {
                // count the number of characters that might be a Shift_JIS single-byte Katakana character
                if (!lastWasPossibleDoubleByteStart) {
                    maybeSingleByteKatakanaCount++;
                }
            }
            if (!lastWasPossibleDoubleByteStart &&
                    ((value >= 0xF0 && value <= 0xFF) || value == 0x80 || value == 0xA0)) {
                canBeShiftJIS = false;
            }
            if ((value >= 0x81 && value <= 0x9F) || (value >= 0xE0 && value <= 0xEF)) {
                // These start double-byte characters in Shift_JIS. Let's see if it's followed by a valid
                // second byte.
                if (lastWasPossibleDoubleByteStart) {
                    // If we just checked this and the last byte for being a valid double-byte
                    // char, don't check starting on this byte. If this and the last byte
                    // formed a valid pair, then this shouldn't be checked to see if it starts
                    // a double byte pair of course.
                    lastWasPossibleDoubleByteStart = false;
                } else {
                    // ... otherwise do check to see if this plus the next byte form a valid
                    // double byte pair encoding a character.
                    lastWasPossibleDoubleByteStart = true;
                    if (i >= bytes.length - 1) {
                        canBeShiftJIS = false;
                    } else {
                        int nextValue = bytes[i + 1] & 0xFF;
                        if (nextValue < 0x40 || nextValue > 0xFC) {
                            canBeShiftJIS = false;
                        } else {
                            maybeDoubleByteCount++;
                        }
                        // There is some conflicting information out there about which bytes can follow which in
                        // double-byte Shift_JIS characters. The rule above seems to be the one that matches practice.
                    }
                }
            } else {
                lastWasPossibleDoubleByteStart = false;
            }
        }
        if (utf8BytesLeft > 0) {
            canBeUTF8 = false;
        }

        // Easy -- if assuming Shift_JIS and no evidence it can't be, done
        if (canBeShiftJIS && ASSUME_SHIFT_JIS) {
            return SHIFT_JIS;
        }
        if (canBeUTF8 && sawUTF8Start) {
            return UTF8;
        }
        // Distinguishing Shift_JIS and ISO-8859-1 can be a little tough. The crude heuristic is:
        // - If we saw
        //   - at least 3 bytes that starts a double-byte value (bytes that are rare in ISO-8859-1), or
        //   - over 5% of bytes could be single-byte Katakana (also rare in ISO-8859-1),
        // - and, saw no sequences that are invalid in Shift_JIS, then we conclude Shift_JIS
        if (canBeShiftJIS && (maybeDoubleByteCount >= 3 || 20 * maybeSingleByteKatakanaCount > length)) {
            return SHIFT_JIS;
        }
        // Otherwise, we default to ISO-8859-1 unless we know it can't be
        if (!sawLatin1Supplement && canBeISO88591) {
            return ISO88591;
        }
        // Otherwise, we take a wild guess with platform encoding
        return PLATFORM_DEFAULT_ENCODING;
    }

}
